Should have said the flight before the Britannia on similar route was on a BOAC B-377 Stratocruiser London -Montego Bay and also NY- LDN in 1958.
I imagine nav-kits and procedures were similar between the two aircraft.
I also forgot to suggest stationed ships/light vessels in support of civil nav operations.
No chance of screwing up like the navigator in Gann's the High and the Mighty! 